1. Field of the Invention
The present invention relates to a method and apparatus in which active material elements are used in injection molding machine equipment (e.g., insert stacks or hot runner nozzle assemblies), in order to provide compression of melt inside a mold cavity, thereby improving the quality of the molded article, including its surface finish and dimensional accuracy, and compensating for plastic shrinkage. “Active materials” are a family of shape altering materials such as piezoceramics, electrostrictors, magnetostrictors, shape memory alloys and the like. In the present invention, they can also be used as sensors.
2. Related Art
Active materials are characterized as transducers that can convert one form of energy to another. For example, a piezo actuator (or motor) converts input electrical energy to mechanical energy causing a dimensional change in the element, whereas a piezo sensor (or generator) converts mechanical energy—a change in the dimensional shape of the element—into electrical energy. One example of a piezoceramic transducer is shown in U.S. Pat. No. 5,237,238 to Berghaus. One supplier of piezo actuators is Marco Systemanalyse und Entwicklung GmbH, Hans-Böckler-Str. 2, D-85221 Dachau, Germany, and their advertising literature and website illustrate such devices. Typically an application of 1,000 volt potential to a piezoceramic insert will cause it to “grow” approximately 0.0015″/inch (0.15%) in thickness. Another supplier, Midé Technology Corporation of Medford, Me., has a variety of active materials including magnetostrictors and shape memory alloys, and their advertising literature and website illustrate such devices, including material specifications and other published details.
Injection compression molding is used in many applications to improve the properties of the article being molded. U.S. Pat. No. 4,828,769 to Maus discloses such an application for molding Compact Discs. U.S. Pat. No. 4,420,454 to Kawaguchi discloses such an application for the molding of a plastic can. Typically, hydraulically actuated cylinders or mechanical actuators are used to induce the final clamping action used to achieve product compression in these examples.
Thus, what is needed is a new technology capable of compression molding that provides a level of adjustable control, and preferably provides embedded sensors and closed loop control of the compression molding process.